1. Field of the Invention
This invention generally relates to dye-sensitive light absorbing chemistry and, more particularly, to a porphyrin molecule useful in dye-sensitive light absorbing applications.
2. Description of the Related Art
Although chlorophyll, chlorophyll derivatives, and synthetic porphyrins have diverse chemical structures, they exhibit similar absorption characteristics over comparable wavelength ranges (typically λ=350-700 nm). Synthetic porphyrins (and their corresponding metalloporphyrins) consist of a conjugated 22π electron system, 18 of which are effectively delocalized to fit the Hückel requirement for aromaticity. In addition to their structural resemblance to natural chromophores such as chlorophyll, synthetic porphyrins are attractive candidates as light-harvesting materials due to their high structural stability, light absorption capabilities in the visible region, redox properties, and synthetic accessibility as compared to naturally occurring chromophores. Photoexcited processes involving porphyrins are facilitated by the highly delocalized π-system, which is capable of resisting major structural changes upon oxidation. Most importantly, the redox properties of porphyrins and metalloporphyrins are dramatically altered upon photoexcitation, which leads to the generation of porphyrin excited states that can be advantageous in photovoltaic (PV) cell applications.
The ability of porphyrins to efficiently harvest light over broad wavelength ranges has generated significant interest in their potential for solar applications over the last few decades. As a result, synthetic protocols towards the fabrication of “customized” porphyrin architectures have become well-established and have been widely adopted as conventional methods. In general, the electronic properties of porphyrins can be readily altered using a number of strategies including the following: functionalization and/or modification along the porphyrin periphery, insertion of transition metals into the macrocyclic core, complexation of metalloporphyrins with various ligands, etc. The strategic manipulation of porphyrin properties (optical absorption characteristics, photoexcited behaviors, etc.) through rational synthetic design has resulted in numerous publications, an overwhelming majority of which are academic in nature.
Most often, the desired enhancements accessible through chemical modification of porphyrins involve manipulation of light-harvesting (absorption) capabilities and/or excited-state behaviors (electron transfer, for example). For example, it is well-known that increasing the pi-conjugation extending from the porphyrin core can lead to enhanced absorption properties which may include (1) increased absorption over a particular wavelength range, (2) a broadening of optical absorption over wider wavelength ranges and/or (3) (bathochromic) shifting of absorption towards longer wavelengths. Furthermore, forward electron transfer processes from photoexcited porphyrin (donor) to acceptor moieties (metal oxides, fullerenes, carbon nanotubes, etc.) can be dramatically enhanced through the strategic introduction of electron transfer facilitating groups into the appropriate locations along the porphyrin core structure. In addition to exerting a favorable influence on electron transfer kinetics, some classes of functional groups can also dramatically improve the overall light harvesting capabilities of the porphyrin, whether it is in terms of increased absorption intensity over particular wavelengths, bathochromic shifting of absorption to longer wavelengths, or both.
At the present, ruthenium(II) bi- and polypyridyl complexes have proven to be the most efficient TiO2 sensitizers in dye-sensitized solar cells (DSSC). However, only incremental improvements in the highest power efficiencies have been achieved within the past decade. Considering the facts that ruthenium(II) pyridyl dyes are expensive and ruthenium itself is a rare metal, there exists significant motivation to develop novel photosensitizers that either contain abundant, inexpensive metals or no metals at all, in response to this, several different classes of photosensitizer molecules have led to appreciably high efficiencies in dye-sensitized solar cells (DSSCs) including indoline (9%), coumarin (5.2%), hemicyanine (5.2%), squarine (4.5%), phthalocyanine (3.5%) and porphyrins (from <1% to as high as 11%). See, respectively, D. L. Officer et al., Coordination Chemistry Reviews 2004, 248, 1363, D. L. Officer et al., J. Phys. Chem., C 2007, 111, 11760, E. W-G. Luau and CA. Yeh et al., Chem. Eur. J. 2009, 15, 1403, C-Y. Yeh and E. W-G. Luau et al., Phys. Chem. Chem., Phys. 2009, 11, 10270, and M. Grätzel et al., Angew. Chem., Int. Ed. 2010, 49, 6646.
FIGS. 1A and 1B are graphs depicting, respectively, the optical absorption spectrum for zinc tetraphenylporphyrin, and quantum efficiency (IPCE) values (%) for zinc tetraphenylporphyrin-TiO2 as a function of optical absorption (prior art). As illustrated, extended π-conjugation in a zinc tetraphenylporphyrin-TiO2 DSSC has previously shown to give rise to high internal photon to current efficiencies (IPCEs) at wavelengths in the 400-700 nm range, see D. L. Officer et al., J. Phys. Chem, C 2007, 111, 11760. Although chlorophyll and its derivatives have lower absorption at λ=500-600 nm (FIG. 1A) relative to λ=450 nm, DSSCs made with zinc tetraphenylporphyrin derivatives still exhibit high IPCE values at λ=500-600 nm (FIG. 1B). In this case, the appreciably high % IPCE between λ=400-700 nm can be attributed to broadened absorption for the zinc porphyrin and a favorable electronic communication between the zinc tetraphenylporphyrin and TiO2 both of which benefit from extended conjugation between the donor and acceptor moieties. Based upon this, it is reasonable to assert the fact that the peak width and absorption edge(s) are the critical parameters when considering the optical absorption spectrum of a potential photosensitizer for DSSC applications. Under standard global AM 1.5 solar conditions, a short circuit photocurrent density (Jsc) of 14.0±0.20 mA/cm2, an open circuit voltage (Voc) of 680±0.30 mV, and a fill factor (FF) of 0.74, corresponding to an overall conversion efficiency of 7.1%, was achieved using this porphyrin photosensitizer. In spite of this, the IPCE values for zinc tetraphenylporphyrin (≈70% at λ=480 and 580-640 nm, respectively) are still lower than that of ruthenium(II) pyridyl complexes. In addition, there is a decrease in IPCE values (≈55%) at λ≈480 nm. Clearly, there exists the potential to improve IPCE values (possibly to ˜80% or beyond).
FIG. 2 is a drawing depicting the molecular structure of a zinc porphyrin photosensitizer (YD1) that exhibits high efficiency (6%) in DSSC when co-adsorbed with chenodeoxycholic acid (CDCA) at ratios of 1:1 and 1:2 (ZnP:CDCA) (prior art). As reported by E. W-G. Diau and C-Y. Yeh et al., Chem. Eur. J. 2009, 15, 1403, this strategy takes advantage of (1) increased conjugation to both broaden and red-shift the absorption characteristics of the photosensitizer and (2) incorporation of a secondary electron transfer “facilitating” group (electron donor) to enhance the electron injection kinetics.
The photosensitizer design (D-P-B-A) takes advantage of a strongly absorbing porphyrin core (P), a conjugated bridge that broadens (red-shifts) the absorption capabilities of the photosensitizer while providing strong electronic coupling (B), a secondary electron transfer “facilitating,” group (electron donor) to enhance the electron injection kinetics from the photoexcited porphyrin (D) and an anchoring group for strong attachment to TiO2 (A).
FIGS. 3A, 3B, and 3C are three analogous porphyrin photosensitizer designs, respectively YD11, YD12, and YD13, based upon the architecture of FIG. 2 (prior art). As reported in the literature by C-Y. Yeh and E. W-G. Diau et al., Phys. Chem. Chem. Phys. 2009, 11, 10270, efficiencies between 6 to <7% were achieved in DSSCs using liquid electrolyte (I−/I3−). Noteworthy is the fact that a DSSC utilizing YD12 as photosensitizer afforded an impressive efficiency of 6.91% versus 7.27% for ruthenium 719 dye (with added scattering layer) within the same cell configuration. The poor performance of YD13 can be reasonably attributed to rapid aggregate-induced energy transfer phenomena due to the presence of the anthracene group in the bridge.
FIG. 4 is a graph depicting the optical absorption spectra for YD11, YD12 and YD13 in ethanol (prior art). Zinc porphyrin photosensitizers YD11-YD13 exhibit the characteristic absorption features for both the Soret Band (400-520 nm) and the lower energy Q-Bands (580-700 nm) with significantly decreased absorption along the regions in between. The broadened and red-shifted absorption for YD11-YD13 is rationalized in terms of the structural design described above, which is a widely known strategy for enhancing absorption characteristics of porphyrins relative to “simple” porphyrins such as pristine zinc tetraphenylporphyrin.
FIG. 5 is the molecular structure of zinc porphyrin photosensitizer YD-2 (prior art). Most recently, M. Grätzel et al., Angew. Chem., Int. Ed. 2010, 49, 6646, have reported an exceptionally high efficiency of 11% for a member of the YD class (YD-2) in a double layer TiO2 film, which is unprecedented for zinc porphyrin photosensitizers in DSSC. To increase the light-harvesting capacity of the devices, an 11 mm (transparent) TiO2 film was coated with a 5 mm thin layer of 400 nm reflecting particles. The IPCE spectrum of the YD-2 device exhibits a broad absorption from 400 nm to 750 nm with an IPCE peak maximum greater than 90% at 675 nm. Jsc (18.6 mA/cm2), Voc (0.77 V) and FF (0.764) were derived from the J-V curve, thus giving an overall power conversion efficiency of ˜11% under illumination with standard AM 1.5 G simulated sunlight.
In light of the recent success using zinc porphyrin-based photosensitizers to achieve high efficiency, it is reasonable to assert that zinc porphyrins have the potential to rival the ruthenium-based dyes traditionally used in conventional DSSCs. In light of this, the rational design of novel porphyrin architectures for DSSC qualifies as an extremely valuable initiative.
Although the strategic introduction of strongly electron-donating di-aryl amine groups at the meso-position of a porphyrin is of interest, the application of aromatic amines to DSSC as co-sensitizers in general, as well the favorable photophysical behaviors obtained by incorporating aromatic amines into photosensitizer core structures, are well established.
Experimental results, reported by Guadiana et al., J. Macromol. Sci, Part A Pure Appl. Chem, 2003, A40, 1295, indicate that electron-donating (aromatic) amines that are anchored to the surface of TiO2 as co-sensitizers effectively enhance the overall photovoltaic performances of DSSCs. Furthermore, they concluded that electron transfer from the amine to the photosensitizer dye is a key step in the photoexcited process.
FIGS. 6A and 6B are, respectively, the molecular structure and optical absorption spectra in dichloromethane of 2TPA, TPA-R and 2TPA-R (prior art). X. Yang, A. Hagfeidt and L. Sun et al., Adv. Fund, Mater. 2008, 18, 3461, report a dye (2TPA-R) containing two triphenylamine (TPA) units connected by a vinyl group and rhodanine-3-acetic acid as the electron acceptor. The experimental results suggest that intramolecular energy transfer processes contributed to the overall light-harvesting abilities of the donor-acceptor (D-A) dye in DSSC. Overall, 2TPA-R exhibited improved photovoltaic performance relative to TPA-R, which is indicative of the favorable enhancements accessible through the careful design of photosensitizers appended with an appropriate aromatic amine containing moiety. In this case, the enhanced performance most likely arises from both a combination of increased optical absorption and enhanced intramolecular electronics.
Although aromatic amines are widely-known to enhance the absorption and/or photoexcited behaviors of photosensitizer dyes, this is by no means a universal generalization. In fact, some ruthenium-based dyes covalently modified with aromatic amines (both conjugatively and non-conjugatively) have failed to produce any real improvements in photovoltaic performance relative to the original dyes (without aromatic amine).
In spite of the strong and broadened absorption for the zinc porphyrin photosensitizers described above, there still exists an overall deficiency in the ability of porphyrins to effectively harvest broad regions of the solar spectrum, especially at wavelengths exceeding 700 nm. Nevertheless, the more recently established potential for porphyrin photosensitizers has positioned this class of materials as a legitimate rival to traditional ruthenium-based dyes for DSSC applications.
It would be advantageous if a porphyrin photosensitizer could be synthesized that performs efficiently in DSSC due to contributions from absorption at wavelengths of 700 um and beyond.